Epoxy Resins

8 High Performance Cycloaliphatic Epoxy Resins for Reinforced Structures with Improved Dynamic Flexural Properties

Downloaded by CORNELL UNIV on October 8, 2016 | http://pubs.acs.org Publication Date: June 17, 1970 | doi: 10.1021/ba-1970-0092.ch008

A. C. SOLDATOS, A. S. BURHANS, L. F. COLE, and W. P. MULVANEY Union Carbide Corp., Bound Brook, N. J. 08805 The combination of tensile strength and modulus of the matrix epoxy resin relate to the edgewise compressive and flexural strengths of the composites, while the combination of tensile strength, modulus, and elongation are reflected in dynamicflexuralfatigue. Epoxy resins with improved toughness have produced laminates with significantly improved fatigue properties under dynamic stressing beyond the levels obtainable with high modulus/low elongation epoxies, by reducing crack initiation and crack propagation. These improvements were obtained by curing the high performance bis(2,3-epoxycyclopentyl) ether/ethylene glycol copolymer with amine hardeners varying in molecular distance between the functional amine groups. T n recent years, there has been considerable progress in the development of higher strength reinforcements for use in structural composites. W i t h the realization that the maximum potential of these higher strength glass, boron, and graphite .filaments could be obtained only with high performance matrix materials, Union Carbide undertook a long range fundamental program under military funded contracts to upgrade the performance of epoxy resins. The major objectives of this investigation were: 1. The development of new types of epoxy resins with cast properties substantially higher than those of the state-of-the-art materials. 2. The development of resin-hardener systems which could be used for the fabrication of practical reinforced structures and which would retain their properties under dynamic stressing, both in air and water. This phase of the program is supported by the U . S. Naval Research Laboratories. 86 Lee; Epoxy Resins Advances in Chemistry; American Chemical Society: Washington, DC, 1970.

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From this work the cycloaliphatic epoxide ERLA-4617 has emerged as the leading candidate, and has, to a large extent, met the above objectives. ERLA-4617, is the copolymer of bis(2,3-epoxycyclopentyl) ether and ethylene glycol, catalyzed with a tertiary amine. The bis(2,3-epoxyclopentyl) ether, designated ERR-4205, shown i n Figure 1 consists of a mixture of liquid and solid isomers, i n a ratio of approximately 35/65.

Figure 1.

Bis(2 3-epoxycyclopentyl) ether ERR-4205; 65% Solids—35% liquid isomers >

E P O N 828 Figure 3.

Epon 828/Epon 1031

Lee; Epoxy Resins Advances in Chemistry; American Chemical Society: Washington, DC, 1970.

88

E P O X Y

RESINS

ERLA-4617 is a very low viscosity material (80-100 centipoises at 25 °C.) with an epoxy assay of 113-120 grams/equivalent of epoxide. The chemistry of this resin, including a postulated reaction mechanism, was discussed i n detail i n a previous paper (1).


The two state-of-the-art epoxy resins used for comparison purposes in this paper are ERL-2772 and E p o n 828/1031; their structures are shown i n Figures 2 and 3 respectively. The typical cast resin properties of ERLA-4617 cured with stoichiometric amount of m-phenylenediamine are compared with the properties of ERL-2772 and E p o n 828/1031 cured with stoichiometric amounts of m - P D A and M N A / B D M A respectively, as shown in Table I. These data demonstrate the substantially higher cast tensile, flexural, and compressive properties of ERLA-4617 compared with those of the state-ofthe-art materials. Table I.

Cast Resin Properties ERLA-2772 m-PDA c

Compressive Modulus, p.s.i. Compressive Strength, p.s.i. Tensile Modulus, p.s.i. Tensile Strength, p.s.i. Flexural Modulus, p.s.i/ Flexural Strength, p.s.i. Heat Distortion Temp., °C. Cure Cycle: 4 hours at 85°C, + a

6

d

8 6 c d v r

Epon 828/l 031 MNA'/BDMA

ERLA-4617 m-PDA

441,000 551,000 890,000 19,200 21,600 32,800 458,000 507,000 783,000 12,900 9,100 19,200 462,000 597,000 815,000 17,500 16,400 31,000 158 143 175 3 hours at 120°C, + 16 hours at 160°C.

ASTM D695-63T. ASTM D638-64T. ASTM D790-66. ASTM D648-56. m-Phenylenediamine. Methyl nadic anhydride.

The work with glass cloth laminates produced from the E R L A 4617/m-phenylenediamine system, using "wet-lay-up" or "prepreg" techniques, demonstrates that the high cast resin properties are indeed translated into high performance composites. The properties shown i n Table II are from laminates assembled by "wet-lay-up," and those shown i n Table III are from prepregs uncatalyzed and catalyzed with B F • M E A . The fabrication and properties of these ERLA-4617 glass cloth composites were discussed i n a previous presentation. 8

Retention of Properties Under Dynamic Fatigue Conditions. A n important prerequisite to practical design of advanced composite structures, i n addition to the need for initially high mechanical properties, is


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retention of these properties under dynamic fatigue stressing. Therefore, a major objective of the present investigation was the development of matrix resin systems which would provide high retention of composite properties under simulated use conditions. Table II.

Properties of "Wet-Lay-Up" Laminates, 181-S-994-HTS Cloth


ERLA-2772 m-PDA

ERLA-4617 m-PDA

Edgewise Compressive Strength, p.s.i. 65,000 Modulus, p.s.i. 3,950,000 Flexural Strength @ 75°F., p.s.i. — Modulus @ 75°F., p.s.i. — Cure Cycle: 16 hours at 85°C. and + 16 hours at 160°C. Table III.

85,000 4,380,000 140,900 4,360,000

ERLA-4617/w-PDA Laminate Properties with and without Catalyst ERLA-4617/m-PDA 1.5% BFMEA b

ERLA-4617/m-PDA

a

Edgewise Compressive Strength, p.s.i. Flexural Strength, p.s.i. Cure Cycle

88,400 149,000 16 hours at 85°C. + 16 hours at 160°C. 32 hours total

87,200 124,000 l h o u r atllO°C. + 2 hours at 120°C. 4- 2 hours at 160°C. 5 hours total

Prepreg uncatalyzed. Prepreg catalyzed with BF MEA. Based on the epoxy resin.

a ft c

0

3

Under dynamic stressing, in contrast to static loadings, stationary cracks in the composites can propagate rapidly, and the toughness of the matrix system should be a measure of its ability to resist extension of the crack. The edgewise compressive and flexural strengths of composites under static loadings increase with increasing cast resin tensile strength. Glass cloth laminates based on the ERLA-4617/m-phenylenediamine system exhibit edgewise compressive strengths at the level of 85-88,000 p.s.i., compared with state-of-the-art bis A-diglycidyl ether at 65,000 p.s.i. as shown i n Tables II and III. The elongation of the E R L A 4617/m-PDA system is approximately 2.5%. Improvements in elongation, therefore, while retaining high tensile strength and modulus (greater toughness), was thought to be the most promising route to providing composites with very high retention under dynamic fatigue in both air and water.


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RESINS

Effect of Amine Hardeners. Throughout the earlier work, ra-phenylenediamine ( m - P D A ) was used as the hardener for ERLA-4617. Recently, this investigation was extended to include other diamine hardeners varying in molecular distance between the active amine groups in an attempt to improve further the cast resin and composite properties. The amines tested were m-aminobenzylamine ( m - A B A ) , methylenedianiline ( M D A ) , and a long chain aliphatic diamine designated Z Z L 0822 (Union Carbide proprietary hardener).


Their chemical configurations and the molecular distance between the active sites, expressed in angstroms, are shown in Figure 4.

Meta-phenylenediamine

Meta-aminobenzylamine

Distance between active nitrogen sites-5.2 A.

Distance between active nitrogen sites-7 A.

Long chain aliphatic diamine (ZZL-0822) Methylenedianiline Distance between active nitrogen sites, 15-25 A.

Distance between active nitrogen sites-10.6 A. Figure 4.

Cast Resin

Amine

hardeners

Properties

The cast properties of ERLA-4617 cured with the various amine hardeners are shown in Table IV. These data clearly indicate that most resin properties are a function of the molecular distances between the active sites of the hardener. (1) In tension, the modulus decreases as the distance between the amine groups increases. The highest modulus (820,000 p.s.i.) was obtained with the m-phenylenediamine (5.2A.) and the lowest (550,000 p.s.i.) was obtained with the long chain diamine, ZZL-0822. The



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elongation, however, as might be expected, followed exactly the reverse pattern. ZZL-0822 produced the highest elongation (greater than 12.5% ) and m-phenylenediamine, the lowest (2.8%). The properties obtained with the other two hardeners, m-aminobenzylamine and methylenedianiline, were intermediate to those of m-phenylenediamine and ZZL-0822. The strengths showed virtually no trend with the exception of ZZL-0822 which had a strength of only 14,000 p.s.i. The strength values obtained with other hardeners are considered to be equivalent, for all practical purposes since tensile values normally fluctuate fairly widely, owing to failures initiated by undetected flaws. For example, values for the E R L A 4617/m-PDA system range from 17,500 to 19,200 p.s.i. (2) In flexure, high modulus was again obtained with m - P D A , (860,000 p.s.i.); the lowest modulus and strength were obtained with the ZZL-0822 hardener. (3) The compressive modulus also followed the same pattern. The highest value (890,000 p.s.i.) was obtained with m - P D A and the lowest 600,000 p.s.i.) with ZZL-0822. (4) The heat distortion temperature data suggest that the aliphatic nature of ZZL-0822 and the hybrid nature of the m-aminobenzylamine (one aromatic and one aliphatic amine groups) influence the heat distortion temperature more than molecular spacing. Since modulus and strength, and particularly the heat distortion temperature of the aliphatic diamine ZZL-0822, were significantly lower than those obtained with the other three hardeners, a blend of ZZL-0822 with m - P D A was used to upgrade properties. The cast properties of ERLA-4617 cured with a 70/30 blend of m-PDA/ZZL-0822 (based on amine equivalent rather than the actual amine weight) are shown i n Table I V . As was expected, all of the properties are between those obtained with either m - P D A or ZZL-0822 alone. Glass Reinforced Composites. W e have already shown that glass cloth laminates with exceptional properties (Tables II and III) have been produced from the ERLA-4617/m-phenylenediamine system using both "wet-lay-up" or "prepreg" techniques. Four additional "wet-lay-up" laminates were fabricated based on ERLA-4617/m-phenylenediamine, ERLA-4617/ra-aminobenzylamine, ERLA-4617/methylenedianiline, and ERLA14617/m-phenylenediamineZZL-0822 (70-30 blend). S-994-HTS glass cloth was used. These laminates were prepared i n order to establish whether there is a relationship between the toughness of the resin and retention of mechanical properties under dynamic loadings. Dynamic Flexural Fatigue. The screening test chosen for this study was dynamic flexural fatigue of the composites in both air and water, run on a Baldwin Sonnatag S F - I - O U , 1800 c.p.m. constant stress fatigue machine under standard conditions at room temperature. To date, only flexural fatigue data i n air have been developed.


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E P O X Y

Table IV.


Heat Distortion, Temp., °C. Tensile Modulus, p.s.i. Tensile Strength, p.s.i. % Elongation Flexural Modulus, p.s.i. Flexural Strength, p.s.i. Compressive Modulus, p.s.i. Compressive Strength, p.s.i. % Deformation

%

Figure 5.

RESINS

ERLA-4617 Cast Properties m-Phenylenediamine

m-Aminobenzylamine

170 820,000 18,500 2.8 860,000 31,500 890,000 31,500 7.5

152 700,000 17,900 3.2 780,000 29,500 850,000 30,000 7.0

STRAIN

Cast resin tensile stress-strain curve

Effect of Toughness on Dynamic Flexural Fatigue. The most recently developed data have shown indeed that improvements i n elongation, while retaining high tensile strength, improve retention of properties under dynamic fatique. A resin system with high elongation and high tensile strength should have greater toughness as measured by the energy to break (area under the stress-strain curve). The tensile stress-strain curves for the cast resin systems E R L A 4617/m-phenylenediamine, ERLA-4617/m-aminobenzylamine, E R L A 4617/methylenedianiline and ERLA-2772/m-phenylenediamine are shown in Figure 5. The area under the curve of the ERLA-4617/methylenedianiline system is considerably higher (832 in. lb./in. ) than that of the ERLA-4617/m-phenylenediamine (320 i n . lb./in. ) which was accomplished by increasing the elongation from 2.8 to 6.0% while retaining high tensile strength. The tougher ERLA-4617/methylenedianiline system, i n turn, produced glass cloth composites with flexural fatigue properties superior to the ERLA-4617/m-phenylenediamine system, as shown 3

3


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Cured with Various Amine Hardeners Methylenedianiline

ZZL-0822

m-Phenylenediamine / ZZL-8022 (70/30)

162 610,000 18,800 6.0 690,000 30,000 750,000 27,000 8.0

80 550,000 14,000 >12.5 580,000 17,500 600,000 17,000 —

128 660,000 17,000 4.0 730,000 29,500 840,000 27,000 6.0

C Y C L E S TO FA.wUaE

Figure 6.

Fatigue results on ERLA-4617 ERLA-2772 in air

and

in Figure 6. While the area under the ERLA-2772/ra-phenylenediamine curve is quite large (800 i n . lb./in. ) the performance of this system in laminates, both under static and dynamic conditions, is significantly inferior to either ERLA-4617/m-phenylenediamine or ERLA-4617/methylenedianiline systems. Although the tensile strength of this resin is low, the area under the curve is large owing to the high ( 8 % ) elongation. Therefore, high elongation, per se, is not the answer to increasing laminate performance; the resin must also have high tensile strength and high tensile modulus. 3

Water

Resistance

Fatigue data of composites in water have not been obtained as yet, but substantial indications on the water resistance of the resin/hardener system itself have been obtained by using a simple screening test.


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EPOXY RESINS

Test for Water Resistance. Cast resin cylinders (0.5 inch in diameter and approximately 1 inch long) were boiled for 24 hours in water. Each specimen was weighed before and after boiling. The weight per cent increase was calculated as follows: ... • i_. /yf t Weight after boil—Original Weight Weight % Increase = j——,—. „ . . — X 100 Original Weight


y

Finally, each specimen was allowed to stand in air and examined periodically for signs of cracking or crazing. The results of the boiling water test of ERLA-4617 cured with m-phenylenediamine, m-aminobenzylamine, methylenedianiline and 70/30 m-phenylenediamine/ZZL-0822 blend are shown in Table V . Table V. Cast Resin Cylinders of ERLA-4617 Cured with Various Amine Hardeners, after 24 Hours in Boiling Water Amine Hardener 100% Stoichiometry

m-PDA/ZZL-0822 (70/30) m-Aminobenzylamine m-Phenylenediamine Methylenedianiline

Cure Cycle

24 hrs. @ + 16 hrs. @ 4 hrs. @ + 3 hrs. @ + 16 hrs. @ 4 hrs. @ + 3 hrs. @ + 16 hrs. @ 4 hrs. @ + 3 hrs. @ + 16 hrs. @

100°C. 160°C. 85°C. 120°C. 160°C. 85°C. 120°C. 160°C. 85°C. 120°C. 160°C.

Weight %, Increase

Remarks On Drying in Air

4.8 3.0

One crack after one week. No cracks.

3.0

No cracks.

1.9

No cracks.

The results indicate that water resistance may be related to the type of hardener used. By this test, ERLA-4617/m-ABA and ERLA-4617/mP D A absorbed 3.0% water; E R L A - 4 6 1 7 / M D A absorbed only 1.9%, and none of the specimens developed any cracks. The E R L A - 4 6 1 7 / M D A system appears to be the least sensitive of all in water and is expected to provide composites with high retention of properties under dynamic stressing in water. In summary, it appears that the combination of tensile strength and modulus of the matrix resin relate to the edgewise compressive and flexural strengths of composites, while the combination of tensile strength, modulus and elongation are reflected in dynamic flexural fatigue in air. The improvement in boiling water resistance of the E R L A - 4 6 1 7 / M D A cast resin system w i l l hopefully be translated into high retention of composite properties under dynamic fatigue stress in water.


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Resins for Reinforced Structures 95

Acknowledgment The authors wish to express their appreciation to U . S. Naval Research Laboratories, Washington, D . C . for sponsoring the above investigation. The authors also acknowledge the contributions of C . M . Eichert, R. F . Sellers, and S. G . Smith, Jr., who have been associated with this project. Literature Cited Downloaded by CORNELL UNIV on October 8, 2016 | http://pubs.acs.org Publication Date: June 17, 1970 | doi: 10.1021/ba-1970-0092.ch008

(1) Soldatos, A. C., Burhans, A. S., Ind. Eng. Chem. Prod. Res. Develop. 5, 225 (1967). RECEIVED

May

24,

1968.


Epoxy Resins

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